1. Field of the Invention
The present invention relates to a hybrid cord with a high tenacity and high elongation at break which can be used in rubber products and other expandable elastic products, particularly to pneumatic tires.
2. Description of the Related Art
Currently, fiber-reinforced rubber products have been applied extensively, such as for pneumatic tires, hoses, belts, and bellows. Fibers embedded in the rubber matrix serve as a reinforcing component to support the structure of the rubber product. Many rubber products have the requirement of a high strength and high pressure, while requiring a sufficiently large deformation or expandability to offset the stress. For example, in some aircraft tires, a protective (breaker) layer with high strength and expandable is required on top of the belt layer. When the tire is in the initial stage of expansion under a pressure or load, the tire and the belt layers will undergo expansion to bear the stress, while the protective layer, too, will expand but will not share much of the load. When the expansion becomes large enough, the protective layer begins to share a significant amount of the load so that the tire is protected at high stress. Another example can be found in some elastomeric bellows. These elastomeric bellows require to have a high pressure rating at burst, and can expand adequately before explosion.
Steel wire and para-aromatic polyamide fibers have high strength, but their elongation at break is low. The steel wire's elongation at break is normally 2%-6%, and the elongation at break of pars-aromatic polyamide fibers is normally 2%-5%. They are both suitable for applications requiring high strength and low elongation at break, but not suitable for applications requiring high elongation at break as mentioned above. Nylon and polyester fibers have a very high elongation at break, both of which reach 15%-25%. However, in some applications that require the bearing of a high burst pressure or high load, neither of them has sufficient strength.
In order to solve the problem mentioned above and to meet the industrial demand, researchers have developed various technologies. Disclosed in U.S. Pat. No. 5,271,445 is a wavy or zig-zag-shaped cord technology. During the initial load stage, the wavy cord is extended to a straightened form, and then during the later load stage, the straightened cord bears the load. An example of the cord material is an aromatic polyamide. A similar technology for the use of steel wire cord is disclosed in European Patent 0567334B1.
Twisting fibers of high tenacity to a high twist level, that is, a high number of sts per unit length, can increase the elongation at break of the fibers and improve the fatigue resistance of the fibers. However, the strength of said fibers also decreases with the higher twist level. For example, Kevlar® 29 fibers of 1500 denier normally have the tenacity of 23 grams/denier and the elongation at break of 3.6%. When it is twisted to 480 tpm (twists per meter), its elongation at break is increased to 5.2%, but its tenacity decreases to 10 grams/denier. When it is twisted to an even higher twist level of 610 tpm, its elongation at break is increased to around 8.5%, but its tenacity further decreases to around 8 grams/denier.
Hybrid twisting of fibers having high tenacity with fibers having high fatigue resistance and high elongation at break, too, is a possible solution. The hybrid cord has already been used in pneumatic tires as a reinforcement material to improve the fatigue resistance and other properties. Disclosed in the US patent publication 2009/0090447A1 is a cord of the aromatic polyamide/nylon hybrid construction (1 ply of aromatic polyamide of 1500 denier and 1 ply of nylon of 1260 denier twisted together). Said hybrid cord has a break load of 332 Newton, which is equivalent to a tenacity of 12.3 grams/denier, and an elongation at break of 16.4%. This hybrid cord can be used in aircraft tires. Disclosed in U.S. Pat. No. 6,799,618B2 are some aromatic polyamide/nylon hybrid cord structures, whose maximum tenacity can be up to 13.3 grams/denier, while their maximum elongation at break can be up to 17%. There have not been any hybrid cords of even higher elongation at break disclosed in the two references mentioned above.
Disclosed in U.S. Pat. No. 4,333,507 is a composite cord for reinforcing tires. The core yarn of said composite cord is polyolefin fibers, such as polyethylene or polypropylene fibers, while its wrapping yarn includes nylon, aromatic polyamide, steel wire, etc. When a composite cord of this kind is embedded in the rubber matrix, because its core yarn can be melted or degraded at the vulcanization temperature of rubber (around 150° C.), the rubber product prepared has a very high elongation at break, but its tenacity is still insufficient. Besides, composite cords made of fibers with high melting points such as a polyamide, polyester, and/or aromatic polyamide, cannot be melted or degraded at the vulcanization temperature of rubber, to provide the enhanced elongation at break.
Therefore, there is still a need to have a hybrid cord of high tenacity and high elongation at break, to apply to rubber products and to other expandable elastic products.